2. Exclusion Criteria: Mothers with pre-existing calcium, uncontrolled thyroid disease, parathyroid conditions, or who require chronic diuretic or cardiac medication therapy including calcium channel blockers will not be eligible for enrollment into the study. Mothers with pre-existing sickle cell disease (not trait only), sarcoidosis, Crohn's disease, or ulcerative colitis may not participate in the study. In addition, because of the potentially confounding effect of multiple fetuses, mothers with multiple gestations will not be eligible for participation in the study. A sub-group of approximately 100 subjects with known diabetes, hypertension, HIV, or morbid obesity (body mass index > 49) will participate in the study.